Tenon size

[Alan Weinberg]

I’ve got less than a year of turning experience. I’ve turned bowls with a mortise and bowls with a tenon. I’ve gotten away with a fairly small diameter mortise as long as there was enough wood surrounding.

I thought that I’d try a small tenon today and just hollow “gently.” I’ve read the rule of 30-40% bowl diameter. This one would be less than 20%. It was a fairly wet (heavy) piece of walnut and I rough turned the outside, put a tenon on, and flipped it. Nice dovetail and didn’t bottom out. Made sure I oriented the grain correctly per other readings and started to hollow. Did not take long to break the tenon. I was fortunately not in the line of fire. Not even a catch.

I remounted on the screw chuck and turned a BIG tenon, bigger than needed. No problem hogging it out. I guess I can always reduce it when I flip the bowl around to work the bottom after hollowing.

Lesson learned. Go big.

 

[Kent Reisdorph]

I've been turning about the same amount of time you have. I've stuck with the same jaws thus far. My tenons are 2". I've had plenty snap off, but usually due to a catch (early on, not much these days) or punky wood. There are also cuts, especially on the inside, that put a lot of pressure on the tenon. Try to avoid those. Before I got my Super Nova 2, my little Wen chuck's tenons were only about 1.25". That's too small for me, but it worked well enough for months until I got the Nova. When I get my new lathe and start turning bigger pieces, I'll use a larger tenon.

 

[Alan Weinberg]

I think the heaviness of this wet wood was a factor. Also, it’s less likely to break if pressure is directed toward headstock and tailstock support is used. I wasn’t using tailstock support and I was directing pressure somewhat laterally just inside rim.

 

[Kent Reisdorph]

I think the heaviness of this wet wood was a factor. Also, it’s less likely to break if pressure is directed toward headstock and tailstock support is used. I wasn’t using tailstock support and I was directing pressure somewhat laterally just inside rim.

Yep, that would be one of those high pressure cuts.

 

[robo hippy]

I think the 30 to 40% rule is pretty good, however I still prefer a recess. You can get away with a smaller tenon, but one catch or one heavy cut, and your piece can go flying. Bigger never hurts. They do not need to be as deep as the one you show. It might have been Stuart Batty who commented that a deeper tenon, even if it does not bottom out, can be weaker than a shorter one. The problem is that the sides of the tenon must match the chuck jaw angle EXACTLY. If not, then there can be a wiggle factor which can cause the grip to fail. I do use a tenon when doing end grain, at least most of the time.

robo hippy

 

[hockenbery]

I thought that I’d try a small tenon today and just hollow “gently.” I’ve read the rule of 30-40% bowl diameter. This one would be less than 20%. It was a fairly wet (heavy) piece of walnut and I rough turned the outside, put a tenon on, and flipped it. Nice dovetail and didn’t bottom out.

A catch will often break a tenon that would do quite well without the catch.

A flat that rests on the tops of the jaws will keep the bowl from rocking back and forth enough to break the tenon.

I set the rim and shape the tenon and the foot early in the roughing process with the bowl gouge. Then connect the foot to the rim with a pleasing curve.
A 2.5” tenon on an 11” bowl. . The flat to rest on the jaw tops


I like a the smooth surface a spindle gouge leaves on the tenon and the flat
Here I smooth the bowl gouge flat with a 3/8 spindle gouge

Bigger tenon is always stronger!

 

[Dave Landers]

That smaller tenon can be ok sometimes, if you can't go bigger.
But wood wants to break between growth rings - that's what makes splitting firewood possible with a maul or wedge.
A tenon on the bottom of the bowl can let the chuck jaws act just like a wedge along that grain, and can split the tenon right off.

The tenon shape is a factor - if the tips of the dovetail jaws engage too much (i.e the tenon is too straight), the tips'll dig in before you get the rest of the jaws tightened, and there's your wedge. Other mismatched tenon shapes can have similar effects.

Over tightening is another culprit - same sort of thing - wedgie. Having a tenon at perfect-circle diameter helps get better grip without having to crank down on the chuck key.

Wood, species, dryness, and grain alignment with the tenon, etc - these are also factors - some pieces just want to split more than others. Same tenon etc might work fine until you get to that "special" piece.

A good defense is what you've just learned: Just knowing it might happen will make you more careful about things like tailstock pressure, aggressive cuts, chuck tightening, etc.
But the best defense is to use those bigger jaws.

 

[Dean Center]

What Dave said. For me, I think one cause of frequent broken tenons was making too much angle on the dovetail. The opposite of what Dave mentions, but I believe it also causes problems. Making a dovetail tenon when using the Nova 'beaked' jaws seems to be an issue for our high school students.

 

[Dave Landers]

making too much angle on the dovetail. The opposite of what Dave mentions

Yeah, I can see how that would cause the jaws to torque/peel the tenon off

 

[Darryl Fective]

There's a lot of fuzz. You need some sharpening, big time.

 

[Alan Weinberg]

Bigger tenon is always stronger!

your skill set is far ahead of mine with my few months turning. I know one can “get away” with a smaller tenon but until I get better, I’ll go a bit bigger. For the record, this time I had no catch.

Thanks.

 

[Alan Weinberg]

Having a tenon at perfect-circle diameter

I know that’s ideal but since this was wet wood that I was rough turning, I knew it would dry out of round and I’d have to return the tenon—so I made it a bit bigger in anticipation of that—was that wrong? I feared that if I made it near circle, when I returned, the jaws would not grip. What is right?

I may have overtightened.

 

[Alan Weinberg]

this one worked out well today but wood was dry and it weighed much less. Love the big Axminster Chuck.

 

[Dave Landers]

I know that’s ideal but since this was wet wood that I was rough turning, I knew it would dry out of round and I’d have to return the tenon—so I made it a bit bigger in anticipation of that—was that wrong? I feared that if I made it near circle, when I returned, the jaws would not grip. What is right?

I may have overtightened.

Your thinking about the size was ok. I was just listing a number of things I've noted as factors when I've sheared off a tenon.

You usually do have to make a tenon over-large when turning wet wood. When twice-turning, it needs to be larger for the same reason you leave the bowl thick (around 10% of the diameter). To put it back on the same chuck jaws, you need to turn away the oval bits to get down to a circle again.

One option is to use larger chuck jaws for the wet turn, and turn the tenon down for the next smaller jaws when dry. Also gets the advantage of larger jaws for the roughing out. Not always possible if you don't have appropriate jaw sets for that bowl. And might not work with your design.

It's all tradeoffs and compromises with wood.

 

[Bill Boehme]

It looks like the chuck might have been overtightened judging from the compression marks, but the greater concern as you noted is that the tenon will be too small for the chuck jaws after the bowl is dry and and the tenon is trued up.

You didn't mention what tool you were using, but from the deep tool marks and grain tear-out, I would guess that you were scraping with a dull tool and applying excessive pressure to compensate for the dull tool not cutting. In the lower right, it appears that the tool might be digging in a bit. If so, it could be on the hairy edge of getting a catch. However, compared to dry wood, wet wood is usually pretty forgiving of technique.

 

[Alan Weinberg]

I would guess that you were scraping with a dull tool and applying excessive pressure

I think you are right. Was using a round nose scraper and probably was dull. Thanks for analysis. I need to sharpen more often.

And what problem does over tightening cause and why—or how? I probably do that as well.

 

[Brian Deakin]

A good source of infromation is

Stuart Batty Tools

stuartbattytools.com

If you scroll down you wil see a 13 minute video

7 Fundamentals - Tenons​

 

[robo hippy]

The overtightening 'can' cause problems, and with both recess and tenon. You need it 'snug', but not as tight as you can get it. The wood can contribute to this as well since some are more brittle than others. If the angles don't match very closely, then you can actually blow a tenon or recess apart when tightening. Same is true if the recess or tenon is not sized properly for the bowl you are turning. Stress on the mount can also be fine, until you get even a tiny catch, which can serve as the straw that broke the camel's back. I do all of my bowl roughing with scrapers since I consider them to be the most efficient tool. I have learned through trial and error, that I can rip bowls off the lathe if I am trying to remove more wood than the mount can handle. Part of why I never use scrapers wider than one inch. The mount can only handle so much metal into the wood at one time before your mount fails. ALWAYS stand out of the line of fire!

robo hippy

 

[Bill Boehme]

I think you are right. Was using a round nose scraper and probably was dull. Thanks for analysis. I need to sharpen more often.

And what problem does over tightening cause and why—or how? I probably do that as well.

I highly recommend learning to use a bowl gouge and make bevel-rubbing cuts that slice the wood fibers rather than shredding the wood. If you can't find a mentor, another option is to use a Hunter carbide tool that can be used effectively in cutting or scraping. I wouldn't recommend Easy Wood carbide scraping tools for wet wood, but they are OK for hard dry wood.

Reed Gray (AKA, robo hippy) gave you an excellent answer on overtightening chucks. Also, he is an expert turner when it comes to using a scraper. I don't recommend beginners staying in their comfort zone by using HSS scraping tools. The reason is that learning to use bevel rubbing tools (skew, spindle gouge, bowl gouge) makes turning an enjoyable experience (perhaps even therapeutic) rather than the nerve-racking anticipation of the next catch-from-hell when barely scraping by.

 

[Kent Reisdorph]

I think you are right. Was using a round nose scraper and probably was dull. Thanks for analysis. I need to sharpen more often.

Alan, shear scraping with a swept-back bowl gouge is a technique that, in my limited experience, is far superior to using a round-nosed scraper on the outside of the bowl. The round-nose is fine for the interior.

 

[Alan Weinberg]

I highly recommend learning to use a bowl gouge and make bevel-rubbing cuts that slice the wood fibers rather than shredding the wood.If you can't find a mentor, another option is to use a Hunter carbide tool that can be used effectively in cutting or scraping. I wouldn't recommend Easy Wood carbide scraping tools for wet wood, but they are OK for hard dry wood.

I use gouges, have and use a Hunter carbide tool, have a mentor, and was trying to use a scraper because that’s what I read Reed uses. I was roughing bowl and not worried re finish. Just trying other stuff.

 

[Darryl Fective]

I think you are right. Was using a round nose scraper and probably was dull. Thanks for analysis. I need to sharpen more often.

And what problem does over tightening cause and why—or how? I probably do that as well.

The over tightening gets back to tenon size. If it's larger than the closed chuck, the corners of the jaws dig into the wood fibers. There's no solid tight, but more of a mushing of wood fibers. Pascal Oidet did a demo and his piece could turn freely in the chuck, but be solid tight for turning with 1/4 turn of the chuck key. When the whole jaw is in contact with the wood, you can't keep going and there's no crushing of fibers.

 

[robo hippy]

Kent, my preferred tool for shear scraping is a scraper with a burnished burr on it. That is a sharper edge, and very durable. I use a spear point scraper on the outside and a round or ) nose shape on the inside. If I had any swept back ground gouges, I would be tempted to use them, but the burnished burr is better and lasts longer.

robo hippy

 

[Rusty Nesmith]

I size my tenon for the size foot I am going to want on the bottom.

 

[Gabriel Hoff]

It looks like the chuck might have been overtightened judging from the compression marks, but the greater concern as you noted is that the tenon will be too small for the chuck jaws after the bowl is dry and and the tenon is trued up.

You didn't mention what tool you were using, but from the deep tool marks and grain tear-out, I would guess that you were scraping with a dull tool and applying excessive pressure to compensate for the dull tool not cutting. In the lower right, it appears that the tool might be digging in a bit. If so, it could be on the hairy edge of getting a catch. However, compared to dry wood, wet wood is usually pretty forgiving of technique.



View attachment 60269

I would also suggest taking a sharp bowl gouge and riding the bevel from the tenon to the rim. Cutting in this direction can get you a tear out free surface.

 

[Andrew Bennett]

Walnut is weak. Don't ask too much of it. If it were me, I'd glue a waste block of harder wood (maple, oak, etc) to the bottom to use as a tenon.

Also, what others have said about all the tearout. Use a sharp tool and a slicing cut. The force exerted on the piece by cutting will be much less.

 

[Kent Reisdorph]

Kent, my preferred tool for shear scraping is a scraper with a burnished burr on it. That is a sharper edge, and very durable. I use a spear point scraper on the outside and a round or ) nose shape on the inside. If I had any swept back ground gouges, I would be tempted to use them, but the burnished burr is better and lasts longer.

robo hippy

I watched one of your Big Ugly videos. Amazing. Your use of scrapers is impressive.

I wasn't aware of the spear point scraper. I have a HSS blank from Amazon not doing anything. Perhaps I should make one.

 

[robo hippy]

I had a spear point scraper many years ago and didn't like it. I was not shear scraping at that time. Can't remember who was the 'signature' on it. Maybe Raffen. You can shear scrape with just about any scraper. I got a bunch of blanks from D Way to experiment. Settled on round nose, spear point, and ) nose. I prefer the ) nose for inside bowls, but round nose works too. Been using the round nose on my adventures with spheres lately....

robo hippy

 

[Bill Boehme]

I use a spear point scraper on the outside

@robo hippy, Is that something the Sorby parting/bead-forming tool?

 

[robo hippy]

Well, some what similar, Mine is 1 1/4 wide, and a spear point like this one, but it has a 65 degree bevel on each side of the point rather than 90 degrees like that one appears to be. Have to see if I can find a picture of one...

Found one from Packard. First one I got was a Richard Raffen one I think...

Packard Woodworks: The Woodturner's Source: Spear Point

robo hippy

 

[Jerry Mollenhauer]

I have learned that for me, while a mortise can eliminate an extra step sometimes, I have to be careful to not overtighten the jaws so as to not crack the surrounding wood, and break. the material loose. I hate having to duck the blowby!

 

[Jerry Mollenhauer]

It could just be that unless my bowl or workpiece has a fairly flat bottom that will support a mortice, I have more faith in using a tennon, especially after I ginned up a homemade vacuum chuck that I can run off my homemade vacuum wood dryer. Then the extra step of tennon removal is a bit simpler than cole jaws.

 

[hockenbery]

Then the extra step of tennon removal is a bit simpler

If you are looking for Simpler yet - consider using a friction drive and the tailstock.

Bowl gouge removes most of the tenon


3/8 spindle gouge turns the nub into a cone


Cutting the cone smaller


The last thing is to cut the tip of the cone free. I do this by turning the bowl by hand


The tiny nub sands off in seconds with 220. Or 180.


You can see the whole process in the returning the dry bowl in the tips

Working with green wood

The August 2015 AAW JOURNAL has great article by David Ellsworth on working with green wood. This is followed by 11 marvelous ways turners turned cracks into features. One of David's themes is that the deck is stacked against newer woodturners successfully working with wet wood. Might be a nice...

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